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Switching generic-to-generic levothyroxine does not change serum TSH

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Levothyroxine is one of the most commonly prescribed medications in the United States, and it is produced by many different manufacturers as generic or brand name products. However, data shows that that there are less prescriptions written for generics than for brand names when compared to other types of medications. Pharmacies in the United States are allowed to substitute levothyroxine by any manufacturer unless a manufacturer and/or “dispense as written” is specifically indicated by the prescriber. The FDA has indicated that generic levothyroxine products at the same dosage are equivalent. However, the American Thyroid Association recommends measuring TSH after a change in levothyroxine manufacturer to make sure that thyroid hormone levels remain stable.

The investigators in this study compared data from patients who consistently received generic prescriptions from the same manufacturer and data from patients who switched between the three most frequently used generic manufacturers to help understand whether there are significant differences in the stability of thyroid hormone levels between those groups when such a change happens.

Review of: Brito JP et al 2022 Association between generic- to-generic levothyroxine switching and thyrotropin levels among US adults. JAMA Intern Med 182:418–425. PMID: 35226058.

Claims from a large database, the OptumLabs Data Warehouse, were evaluated. This included commercially insured and Medicare Advantage enrollees throughout the United States. Included were adults who filled a prescription for generic levothyroxine made by Mylan, Lannett or Sandoz between January 1, 2008 and June 30, 2019, had a stable dose, the same manufacturer and a normal TSH for at least 3 months before either continuing the same product or switching to a different generic product.

There were 15,829 patients who filled generic prescriptions for levothyroxine. Most patients were female (73.4%), average age was 58.9 years, and received a dose of levothyroxine of 50 mcgs or less/day. The generics manufactured by Mylan were 56.8% , Lannett 34.4% and Sandoz 8.9% of prescriptions. The investigators then designated two groups of patients. The “non-switchers” continued with the same manufacturer (13,049 patients); the “switchers” changed manufacturers (2780 patients). These last group was matched 1:1 with 2780 non switchers based on TSH level, age and medical problems.

There was no significant difference in the proportion of patients achieving a normal TSH between switchers (82.7%) and non-switchers (84.5%). The results were consistent even when the group that was on higher doses of thyroid hormone was assessed.

This study shows that changing generic manufacturers did not result in a significant change in blood levels of thyroid hormones. It is reassuring to know this, since there is a significant price difference between brand name products and generics. What is not known, and should be studied, is whether switching back and forth between generic and brand name levothyroxine would lead to the same results. In any event, it is helpful for both physicians and patients to know that switching between generic manufacturers is not a cause of variable thyroid blood levels.

— Jessie Block-Galarza, MD


Levothyroxine (T4): the major hormone produced by the thyroid gland and available in pill form as Synthroid™, Levoxyl™, Tirosint™ and generic preparations.

TSH: thyroid stimulating hormone – produced by the pituitary gland that regulates thyroid function; also the best screening test to determine if the thyroid is functioning normally.

Generic levothyroxine: made by Mylan, Lannett or Sandoz

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